The containment, or storage, of large volumes of gases or of liquids with relatively high vapor pressure is economically possible only in underground caverns sufficiently deep so the overburden will allow the containment of the product in a pressurized state, while motivation of stored product in and out of storage can be accomplished without fear of fracture or loss through leakage into an overburden. Stored product is often hydrocarbons, and these are frequently gaseous at atmospheric pressure and ambient temperature, although they are most frequently in liquid condition in storage. Stored hydrocarbons include, for example, propane, butane or mixtures, including ethane and propane mixtures. But the product can be consistently liquid, for example crude oil in storage.
For convenience, these underground storage regions will frequently be referred to herein simply as "caverns." Such caverns can be prepared solely for containment or storage purposes, or may have had previous use, e.g., brine production wells. In such wells, stored product can be a hydrocarbon which, due to difference in specific gravities, supplants part of the brine in the cavern. For hydrocarbon removal, brine or water is introduced into the well, thereby forcing out the hydrocarbon.
For convenience in the discussion herein, the term "hydrocarbon" will be frequently used, but is to be understood as illustrative only, since other products are contemplated for storage or disposal, as will be more particularly discussed hereinbelow. An entry configuration, or passageway, to the cavern can contain a casing arrangement that will include a barrier casing, or "cemented" casing, that can be sealed, e.g., cemented, along its outer surface from the casing seat to the surface. Most typically a liner casing will also be present so that an annulus exists between the barrier and liner casings. As will be discussed more particularly hereinbelow, other inner casings may also be present. Hereinafter, all such casing arrangements will generally be discussed in connection with brine wells, but it is to be understood that such discussions are not meant to be restricted to wells, but are presented by way of illustration only.
In the cavern, e.g., a brine well used for hydrocarbon storage, the annulus between the barrier and liner casings will ususally contain hydrocarbon when the hydrocarbon is less dense than the brine. Since the annulus can be used to transport the hydrocarbon, any leakage can cause a serious problem, such as from economic losses and environmental damage. Moreover, because of the large quantities of stored hydrocarbons, slow leaks may not be quickly evident. Casings from brining operations may be especially susceptible to leakage because of the nature of the operation, i.e., the potential for corrosion due to the nature of the salt leaching operation. It must also be kept in mind that leakage can also occur from the annulus through the liner casing. Further, when the barrier casing does not extend down completely to the cavern roof, but rather to a point above the roof so that a lower borehole section extends to the cavern, leakage may occur at the casing seat or in this region of the completed depth of the barrier casing section which extends to the cavern.
It has been known to add a casing inside the liner casing and then fully block the annulus between the barrier and liner casings by inserting a packer as an obstruction in the annulus at a region above the seat of the liner casing. The annulus above the packer is then completely filled with liquid and pressure maintained on the liquid is monitored at the wellhead. But ambient temperature changes on the liquid system cause pressure fluctuations, even with small volume changes, which are not related to leakage and may therefore be misleading. Also, this arrangement does not allow for detection of leaks either at the barrier casing seat or in the extended borehole section below the seat. Moreover, with liquid filling, this method must rely on a completely blocked annulus.
Heretofore, it has also been known to conduct tests on the barrier and liner casings, prior to cavern use for hydrocarbon storage and at periodic workovers. Such test efforts comprised merely supplying liquid or gas to the cavern and then checking for changes, if any, in the supplied gas pressure under static conditions. If the results were acceptable, the test was completed, the gas removed and the casing arrangement was regarded as suitable for use with the storage of hydrocarbons in the cavern. Otherwise, where possible, it was repaired and retested. If the test could not be passed, the entry would be plugged.
It would therefore be desirable to provide for hydrocarbon storage where the full entry configuration from the cavern through the overburden can be monitored for leakage. It would be moreover desirable to provide a leakage detection system that will detect small leaks and prevent loss of product with associate conservation of natural resources. Through fast detection, associated with loss prevention, any leakage that does occur should have no, or reduced, environmental impact. In caverns or entries into porous strata used for disposal, it would be desirable to protect the annulus from corrosive materials and to be able to control situations wherein leakage may be occurring through the packer blocking the annulus. Further, such leakage detection and prevention arrangement should be adaptable for a variety of casing and equipment arrangements, while not necessarily being dependent on the use of special equipment. Simplicity of such a system, as well as ease in the monitoring of same, would also be highly desirable. The system should have the flexibility to provide for maintenance of sustained operation during small leakage as well as permit hydrocarbon removal for entry modification. It would also be desirable to facilitate replacement of the inner casing, in storage of sour or corrosive product, e.g., sulfur-containing hydrocarbon, when dictated by leak detection, through replacement in planned, preventive or emergency repair.